What question are you answering?:
How is it used?:
"Ever since I read that book about blue Chevies I've seen them everywhere. It must be Baader-Meinhof."
What does it mean?:
Baader-Meinhof is the phenomenon where one happens upon some obscure piece of information– often an unfamiliar word or name– and soon afterwards encounters the same subject again, often repeatedly.
How did it come to be used this way?:
http://www.damninteresting.com, which is the only place I can find that mentions this term in association with this phenomenon, claims that "How the phenomenon came to be known as "Baader-Meinhof" is uncertain. It seems likely that some individual learned of the existence of the historic German urban guerrilla group which went by that name, and then heard the name again soon afterwards. This plucky wordsmith may then have named the phenomenon after the very subject which triggered it." Please note that this may well be made up information, and there may well be another, legitimate name for the phenomenon the questioner described.
Trivia/fun facts:
http://www.damninteresting.com/?p=417
